1. Field of the Invention
The present invention relates, generally, to a method for aligning packet loss priority information in a data-packet-switching communications device and, more specifically, to such a method wherein the packet loss priority information may be modified depending on a connection type or an application-specific data traffic type and the original packet loss priority information is restored after a data packet has been switched in a respective communications device.
2. Description of the Prior Art
In existing and future packet-oriented communications networks, different monitoring strategies are and will be provided to monitor variable and defined data packet transmission rates. Particularly in communications networks operated on a cell-oriented basis, for example ATM communications networks operating according to the Asynchronous Transfer Mode, loss priorities are allocated to ATM cells which are to be transmitted and, with reference to the loss priorities, a decision is made in a communications device concerning the further switching of the respective data packet. On the basis of the allocated loss priorities, a decision is made in the respective communications device with the aid of a monitoring procedure; in particular, concerning the further switching or rejection of an ATM cell. The data packets which can be rejected within the ATM communications network in the event of overload without the loss of real-time-related, connection-individual information, thus can be defined via the packet-individual allocation of loss priorities.
Furthermore, different traffic classes or connection types are defined in the proposal entitled “Traffic Management 4.0” of the ATM Forum 1996. These include Constant Bit Rate (CBR) connections, Variable Bit Rate (VBR) connections, Available Bit Rate (ABR) connections and Unspecified Bit Rate (UBR) connections. The Constant Bit Rate connection type is used for virtual connections, for which a defined transmission bandwidth must be continuously provided for the duration of the virtual connection. The Constant Bit Rate connection type is therefore used, in particular, for real-time-related, virtual applications such as voice transmission.
The Variable Bit Rate connection type is defined for virtual connections with variable or changing transmission requirements in the proposal entitled “Traffic Management 4.0” of the ATM Forum 1996. Knowledge of the traffic characteristics of the application represented by the respective virtual connection is advantageous for this purpose. A distinction is made, in particular, between real-time-related and non-real-time-related Variable Bit Rate connections wherein, for example, transmission of real-time-related video data with a variable bandwidth is to be understood as a real-time-related Variable Bit Rate connection. The Available Bit Rate connection type enables applications to which no special transmission bandwidth is allocated. The applications can use the transmission bandwidth which is currently possible in the ATM communications network, wherein a maximum and a minimum transmission rate are allocated, in each case, to the respective Available Bit Rate connection and these limit values must not be exceeded or undershot. On the basis of the usage factor of the respective ATM communications device, the currently possible transmission rate is indicated to the transmission device with the aid of control cells periodically inserted into the ATM cell stream. With the aid thereof, following the arrival of the control cells in the transmission device, the transmission rate of the ATM cells of the respective virtual connection is adapted to the currently possible transmission rate. In the case of the Unspecified Bit Rate connection type, no defined cell loss information or cell delay times are allocated to the respective virtual connection. Instead, the Unspecified Bit Rate connection type represents a “best-effort” service class which is provided in practice, for example, for Internet applications.
The loss priorities allocated to the respective ATM cells of a virtual connection, i.e. the cell loss priority information transmitted in an external data packet header with the data packet, are evaluated during the switching of the individual virtual connections within an ATM communications device, depending on the connection type. The connection type of the respective ATM cell is thus initially defined and, following alignment of the connection type priority with the loss priority of the respective ATM cell, a decision is made with the aid of the monitoring procedure concerning the forwarding or rejection of the ATM cell. The data packets are then further processed or switched in the ATM communications device with the aid of the switching elements, inter alia on the basis of the cell loss priority information recorded in the external data packet header.
Two connection types, the Constant Data Rate connection type and connections with a low loss priority, have primarily been taken into account in known and practically relevant methods for aligning cell loss priority information. According to the definition of the aforementioned connection classes by the proposal entitled “Traffic Management Specification 4.0” of the ATM Forum, the newly added connection classes must be taken into account in existing data-packet-switching communications devices and the loss priorities which differ according to the connection type must therefore be aligned with the existing communications devices; i.e., in particular with their switching networks. In the known methods, particularly those relating to Constant Bit Rate connections, a check is carried out by the communications device or its switching elements on the cell loss priority information, wherein a low loss priority is allocated as standard to Constant Bit Rate connections so that these connections are never rejected in the event of overload. In contrast to this, in the case of virtual connections with a high loss priority, for example Variable Bit Rate connections, the associated ATM cells are rejected within the communications device in the event of overload.
An object of the present invention, therefore, is to improve the alignment of packet loss priority information for overload control of a data-packet-switching communications device.